Crown rump length

Crown rump length (CRL) is the length of the embryo or fetus from the top of its head to bottom of torso. It is the most accurate estimation of gestational age in early pregnancy, because there is little biological variability at that time.

Measurement

CRL is measured as the largest dimension of embryo, excluding the yolk sac and extremities. It is used as a primary measure of gestational age between 6-13 weeks. After 13 weeks, head circumference, biparietal diameter, and femur length measurements become more useful measurements for assessing fetal growth.

Practical points

The earlier in pregnancy a scan is performed, the more accurate the age assignment from crown rump length ⁴. If the original CRL measurement was adequate, the measurement is considered the baseline for all subsequent age measurements.

Overall, the accuracy of sonographic dating in the first trimester is +/-5 days (95% confidence range).

Cardiac activity should be present in an embryo with a CRL ≥7 mm ³. On a transvaginal scan performed by an experienced operator, absence of cardiac activity in an embryo with CRL ≥7 mm is diagnostic of a failed early pregnancy (missed miscarriage).

It has been reported that patients in whom MSD (mean sac diameter) is less than 5 mm greater than crown rump length (i.e. MSD - CRL = <5 mm) are prone to first trimester miscarriage, despite a normal heart rate.

Chromosomal anomalies, particularly trisomy 18 and triploidy are markedly associated with growth restriction, i.e. decreased crown rump length.